The present invention relates to an optical information medium which is capable to record/reproduce an information optically, especially for being capable to obtain a sufficient signal.
Two kinds of tracking servo method are known, one of which is a continuous groove servo method, and another of which is a sample servo method as shown in U.S. Pat. No. 4,402,061. The continuous groove servo method has already been used in products. The sample servo method is recently developed because of a good tracking stability.
As shown in attached FIG. 1A, the sample servo method does not use guiding grooves for tracking, but use a pair of smaple marks 10, 11 for tracking. The pair of sample marks are located on opposite side of the imaginary track with slight off-set. A center line of the imaginary track is indicated as a chain line. The accurate tracking of the imaginary track has been done by detecting each sample mark. In FIG. 1A, a reference numeral 12 indicates pits for clocks. A reference numeral 1 indicates a replica base plate made of, for example, plastic. FIG. 1A is a typical plan view, and FIG. 1B is a typical cross sectional view along line 1B--1B in FIG. 1A. The length of each of pits 10, 11, 12 is 90 ns on a time axis, which corresponds to 0.5 .mu.m in case of 1800 rpm of revolution number of the optical disc, and pits located at a position of the radius of 30 mm of the optical disc. Optical depth of each of pits 10, 11, 12 is .lambda./4, where .lambda. is a wavelength of a laser beam for recording/reproducing , and 830 nm is normally used. The sample mark portions shown in FIG. 1A are necessary to 1000 to 3000 portions on each imaginary track. In one embodiment, the number of the sample mark portions are selected to 1376 portions on each track. In other area, there are sector address portions which are located 30 portions on each track. The remaining areas excepting the sample mark portions and sector address portions are mirror areas which are used for recording some information or data. Therefore, the area occupied by some pits of sample marks and sector address is about 10% of total area of the optical disc.
The inventors tried to manufacture the optical disc for the sample servo method by normal injection molding technique, but the ghost pits 10a, 11a, 12a shown in FIG. 2 are produced by double transcription for the original pits 10, 11, 12. In a worst case, triple or quardruplex transcription occurred. The double transcription has a bad influence on tracking signal, and causes a problem of abnormal tracking. In FIG. 2, imaginary center lines of tracks are omitted.